Primary breast tumors can shed millions of cells into the blood stream, known as circulating tumor cells (CTCs), at early stages of tumor progression. CTCs are able to survive in the circulation and arrest at distant sites to form metastatic disease. The presence of CTCs in a patient with breast cancer is associated with poor prognosis and increased risk of metastasis. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) has anti-tumor activity in breast cancer and is currently being evaluated in clinical trials as a target for breast cancer therapy. However, the role of AMPK in breast cancer metastasis is largely unknown. Most studies have focused on the function of AMPK as a cellular metabolic regulator, but AMPK has numerous other roles including maintaining epithelial cell polarity and regulating cytoskeletal proteins. Disseminated epithelial tumor cells use tubulin-based structures known as microtentacles to adhere to blood vessel walls in order to survive in the circulation and arrest in the capillaries of distant organs. There is a higher frequency of microtentacles in metastatic breast cancer cells, and microtentacles are enriched with a stable, detyrosinated form of a- tubulin. AMPK can phosphorylate the microtubule plus-end binding protein, clip-170, which results in decreased microtubule stability in normal epithelial cells. In this study, we will establish the role of AMPK in stabilizing microtubules in breast cancer. Furthermore, we will investigate the effects of clip-170 phosphorylation on microtubule stability and the metastatic efficiency of detached breast tumor cells. This study will test the hypothesis that AMPK activation decreases microtubule stability, resulting in decreased CTC attachment. We will test this hypothesis in the following specific aims: 1) Determine the extent of AMPK regulation of microtubule stability and microtentacle formation. 2) Define how AMPK phosphorylation of clip- 170 affects CTC attachment and metastasis. Current chemotherapeutics used in breast cancer therapy that target microtubules can enhance microtentacle formation and increase CTC re- attachment. Clip-170 provides a novel therapeutic target for microtubule stability that may reduce the metastatic success of CTCs or act as a biomarker for AMPK therapy. Completion of this study will help determine the therapeutic potential of AMPK-directed drugs to decrease the attachment of CTCs to blood vessels. Therefore, we can take advantage of the existing and developing pharmacological AMPK activators to decrease CTC metastasis and increase survival of patients with breast cancer.